1. Field of the Invention
This invention relates to thin films and more particularly, it relates to epitaxial thin films.
2. Description of the Prior Art
Single crystals of many materials have been grown epitaxially upon a substrate composed of a single-crystal structure. However, the degree of perfection of the films obtained is generally low unless the degree of misfit between the stress-free lattice parameters for the film and the substrate is small. It is possible, for example, to grow perfect films of doped silicon on silicon (where the misfit can be as low as 10.sup.-5) but it is difficult to grow perfect films of GaP or GaAs (where the misfit is about 4.times. 10.sup.-2). When the misfit is small, thin films grown on a substrate will sometimes strain elastically to match the substrate. Examples quoted in "Coherent Interfaces and Misfit Dislocations," Epitaxial Growth, Part B, edited by J. W. Matthews, pp. 572-573, Academic Press, New York 1975, include nickel on copper.... gold on silver....palladium on gold....platinum on gold....cobalt on copper.....alpha.-iron on copper.....alpha.-iron on gold....germanium on gallium arsenide....lead sulfide on lead selenide....and garnet films on garnet substrates.
It is also well known that face centered cubic (f.c.c.) metals tend to form weak bonds with alkali halides and that in a number of cases epitaxially aligned but poor quality thin films can be grown on such substrates despite relatively large misfits (J. W. Matthews op. cit. p. 566). In these circumstances, the growth of the epitaxial film begins with the generation of isolated three-dimensional islands.
Many partial solutions to the problem of growing good quality epitaxial films on a substrate with which the overgrowth has a large misfit have been suggested. Abrahams et al, J. Material Science, Vol. 4, p. 223 (1969) have shown that the growth of a graded alloy layer between a substrate and the ultimate deposite desired can yield a deposit with a fairly low dislocation density. However, it should be emphasized that the resultant defect density in the final film is nowhere near the ideal. It is much higher than the density of defects in the substrate. Also, the method is impractical when the misfit is larger than a few percent because the lattice parameter gradient in the graded region must be small. Thus, when the misfit is large, the thickness of the graded region becomes prohibitively large.
U.S. Pat. No. 3,935,040 of Mason teaches depositing a layer of SiGe alloy epitaxially upon a pure Si single crystal substrate by gradually increasing the Ge concentration from 0% to 8% over a layer thickness of a few micro-meters using vapor phase deposition techniques. Then GaP is deposited upon the SiGe alloy. The lattice constant for Si is 5.42 and that GaP is 5.45, which relatively minimal (0.56%) mismatch is reduced by the above process. The result can again be unsatisfactory if the thickness or electrical properties of the intermediate layers are important.
U.S. Pat. No. 3,661,676 of Wong teaches using small single crystals of Al.sub.2 O.sub.3 positioned and cemented on a polycrystaline aluminum substrate for forming a large single crystal of Al.sub.2 O.sub.3 by catalytic oxidation of AlCl.sub.3 in the presence of water vapor. This method differs from that described here in that the substrate is not a single crystal; the seed crystals are not microscopic, do not have spherical surfaces facing away from the substrare, and are not loosely bonded to the substrate to permit sliding across the substrate. The patent does not indicate the number of deflects in the single crystal produced. There is also no mismatch between the lattice constants of the layers.
U.S. Pat. No. 3,788,890 of Mader and Matthews, entitled "Method of Preparing Dislocation-Free Crystals" describes an epitaxial deposition techanique with no intermediate layer, by means such as vacuum deposition upon a suitable monocrystalline plane.